Steve, what did you go for in the end?
Antec Sonata is the 'off the shelf' silent option. but you can modify any case. Anything with 120mm fans is a good start, avoid windows as these leak sound ~(much like houses really)~ And a decent PSU is also a must. The Antecs are good if you still have yours.
I've just bought a black Lian-Li case, All aluminium so nice and light. Bit too flimsy soundwise though, so I cut a square of 12mm mdf and fixed it inside the side panel to deaden it. Also lined with acoustic foam.
Intel dual cores don't actually run very hot. But you will definitely need at least one fan pulling air through, for the sake of the HDD's.